Atherosclerotic plaque is a relatively common occurrence in these times of rich foods and long life. The plaque produces a stenosis reducing the diameter of the lumen of the artery and restricting blood flow to the region beyond the stenosis. In some instances a balloon catheter may be employed to increase the diameter of the lumen particularly where complete blockage of the artery has not occurred. The balloon increases the diameter of the vessel by stretching the wall beyond the limit of elastic recovery but does not compress plaque that has hardened. In those instances where a balloon catheter cannot be used or cannot be used initially, catheters may be used t bore through the plaque and increase the diameter of the lumen through the stenosis so that if necessary a balloon catheter may be employed.
The use of steerable catheters to remove atherosclerotic plaque both from coronary as well as peripheral arteries is in increasing use today. In the art today, there are heated catheters, cutting blade catheters and various types of laser catheters all of which may be quite dangerous in use since, if aimed incorrectly or overheated, they can damage the wall of an artery producing serious, if not fatal, injury or cause particles to enter the blood stream.
Various approaches to reducing these hazards have been suggested. Very thin guide wires may be employed to facilitate guiding the catheter to the proper location. Although probably not used in practice, some patents disclose feeding a laser through the arteries to remove plaque. To better control various factors such as size of the instrument, aim of the beam and the energy supplied by the laser, optical fibers are now employed to conduct laser energy to the site of the stenosis.
Various of these techniques are being employed today but dangers of overheating and misdirection are still prevalent although the use of guide wires has materially reduced the danger of mechanical puncture of the wall of an artery. Further, there is still the danger that solid debris from the plaque or liquified plaque may enter the blood stream and produce serious blockage, particularly in small coronary arteries.
The use of a heated catheter requires a delicate balance of various factors. It is desirable to concentrate the heat in an axially forward direction at the distal face of the heater of the catheter while maintaining the sides of the catheter at a relatively low temperature. The distal face of the catheter must be able to rapidly reach a temperature that produces vaporization of the plaque. As to the range of allowable temperatures of the sidewall of the catheter, the upper end of the range must be below a temperature at which the catheter will stick to the wall of the artery, or approximately 120.degree. C. Such temperature, however, should be high enough to damage without charring the inner surface cells of the wall so that the rate of future adherence of plaque to the wall will be greatly reduced.
Such control of sidewall temperature of a heated catheter can be achieved by maximizing forward heat flow while carefully controlling flow of heat from the distal face of the heater towards its proximal end and radially from the core of the heater to the sidewall whereby the length of the sidewall behind the head that is at temperatures that can produce sticking is sufficiently short as to not allow sticking during use.